High performance engines for military aircraft utilize augmentation, sometimes referred to as afterburners, for increased thrust. This augmentation injects additional fuel into the exhaust and ignites the fuel, thereby providing additional thrust. The components utilized to provide this thrust are located in the exhaust path of the engine, such as the spray bars and the igniters, and are exposed to the exhaust gases. Of course, the components downstream of the spray bars and igniters, such as the secondary seals, are exposed to elevated temperatures even higher than the exhaust temperatures as a result of the injection of additional fuel into the exhaust stream.
To protect the components in the exhaust stream and those downstream from the spray bars and the igniters due to the elevated temperatures from melting or other deterioration, these components have been coated with a thermal barrier coating system (TBC) comprising a top coat of 8-12 mils (0.008-0.012 inches) of yttrium-stabilized zirconia (YSZ). However, as jet engine temperatures have increased, with a resulting increase in the temperatures in the augmentor portion of the engine, the temperatures to which the coatings have been exposed have increased, so that even the most advanced of these alloys are operating in temperature regimes that can exceed their melting temperature.
Although the augmentors are subject to short duty cycles since the periods for which the additional thrust is required tends to be limited, the thermal loads have been sufficient to cause what appears to be localized melting and associated warpage in the vicinity of the localized melting on the edges of the seals. These thermal loads are the result of heat transferred from the hot gases of combustion in the exhaust both from the combustors and from providing augmented thrust as well as from radiation, typically infrared, when the augmentors are providing thrust (i.e. when the afterburners are lit). Since YSZ is substantially transparent to IR, the YSZ provides little benefit in protecting the substrate from thermal loads from this heat load mechanism. The IR is absorbed by the material underlying the YSZ, typically the substrate, resulting in additional heating of the substrate.
What is needed is a coating for metallic components operating in the hot gas stream of a gas turbine and subject to IR from combustion reduces thermal load on the substrate material so that incipient melting of the substrate is substantially reduced or eliminated. The coating should reduce the thermal load experienced by the substrate by reducing heat transfer from the hot gases of combustion and should reduce or eliminate the penetration of IR to the substrate.